Electromagnetic interference shielding materials are typically formed by incorporating particles of an electrically conductive material such as silver, gold, copper, nickel or the like into a nonconductive polymeric substance. More specifically, to obtain an electromagnetic interference shielding effect, an EMI shielding material must provide a pathway for electrical current by establishing an interconnecting arrangement of the electrically conductive particles. Unfortunately, a high density of the electrically conductive particles must be incorporated into the nonconductive polymeric substance in order to provide the electrically conductive interconnecting arrangement required for electromagnetic interference shielding, thereby resulting in the formation of a relatively weighty (high specific gravity) end product having limited applications. Further, the heavy, electrically conductive particles tend to settle throughout the less dense nonconductive polymeric substance during and after production, oftentimes resulting in a shielding material having a nonuniform electrical conductivity.
Low density, lightweight, electrically conductive magnetic microballoons, each comprising a hollow, porous ceramic shell having a surface coating of an electrically conductive material such as silver or the like, have recently been developed in order to overcome the deficiencies of the above-described electrically conductive, metallic particles. More specifically, a large number of the lightweight microballoons may be incorporated into a nonconductive polymeric substance without adversely increasing the overall weight of the resultant composition. Further, the degree of settling within the nonconductive polymeric substance is greatly reduced due to the light weight of the microballoons. Finally, the mutual magnetic attraction between the metal coated, magnetic microballoons greatly facilitates the formation of an electrically conductive, interconnecting arrangement throughout the nonconductive polymeric substance, thereby forming a substantially uniformly conductive composition which is ideally suited for electromagnetic shielding.
As known in the art, conventional silicone rubber curing methods are achieved using vinyl and various types of catalysts. Unfortunately, I have discovered that when electrically conductive, magnetic, silver coated ceramic microballoons are incorporated into conventional silicone rubber base materials in quantities sufficient to provide electrical conductivity, the resultant composition cannot be cured in accordance with conventional silicone rubber curing methods due to a resultant depletion of the catalysts. As a result, there is a tendency for the base materials to break up before a sufficient quantity of the silver coated ceramic microballoons can be added therein to achieve electrical conductivity and EMI effectiveness.